As a result of environmental laws, waste disposal regulations, economic forces and other factors, considerable research is being conducted in the United States, Europe, Japan and elsewhere in the world to remove substances, particularly heavy metals such as lead, cadmium and mercury, from many products. For example, European Union initiatives have mandated <0.1% Pb in imported articles and products by mid-2006. Such products include consumer products such as televisions; computer monitors, processing units and any peripheral equipment; personal music and video devices (for example, iPOD™ and similar devices); cell phones and other handheld communications devices; and any other device using a display screen or requiring glass (or glass-ceramic materials) as a sealing material. Plasma display screen devices are representative of such devices that use glass or glass-ceramic materials as sealing materials.
Plasma display panels (“PDP”) and devices using plasma displays (“plasma display screen devices”) offer substantial benefits in size, thinness, and viewing angle compared to, for example, current CRT (cathode ray tube) technology. As a result, major growth in these displays is occurring, and is forecasted to continue through 2010. However, concurrent with the projected display growth, there is also a growth in the number of countries improving their environment laws, and particularly their waste disposal laws. As a result, the materials that are used as seals or sealing agent must meet ever tightened environmental requirements. Currently, these displays are sealed with a number of different Pb-containing frits. Lead-containing frits offer the advantages of good flow and good adhesion at moderate sealing temperatures (450-550° C.). However, lead (“Pb”) is viewed as a toxic material, both from the standpoint of workplace safety as well as the viewpoint of its being a landfill contaminant. As of 2006, the European Union through its environmental initiatives WEEE (Waste of Electrical and Electronic Equipment), and RoHS (Reduction of Hazardous Substances) is driving to restrict the allowed level of Pb in imported articles to <0.1% by mid-2006. Other regional associations or countries are discussing similar restrictions.
A frit is a powdered glass that sinters to form a structure that may incorporate features and/or elements, or may simply act as a seal or sealing agent between two materials. The plasma display device consists of two parallel glass plates separated by a precise distance and sealed around the edges. The space between the glass plates is filled with an ionizable gas or gas mixture, for example, a neon-xenon gas mixture, and phosphors (red, green and blue); and parallel electrodes are placed on each glass plate, the electrodes on one plate being orthogonal to those on the other plate. The “intersections” of the electrodes on each plate (which are separated by a precise distance) define “cells” or the individual color elements. In order for the plasma display to function, the individual cells must be separated from one another, which is typically done by inserting a honeycomb-like structure between the two plates. The interior walls of the each cell are coated with a phosphor of a single color. To produce a picture, the gas or gas mixture is ionized by the electrodes, and the ionized gases excite the phosphors which in turn glow the appropriate color. In order for the plasma display to properly operate for a long period of time, the seal created by the frit must be “fluid tight” so that ionizable gases do not escape the devices and oxygen, water vapor and other gaseous substances do not enter the chamber. Further descriptions of plasma devices can be found in U.S. patent/application Ser. Nos. 6,924,597, 2005/0245166 and 6,970,219 among other published art. Examples of lead-containing glass materials that are presently used as sealing materials can be found in an article by Kwon et al., “Plasma display panel vacuum in-line sealing technology using a bubble-reduced frit”, J. Vac. Sci. Technol. A 21(1) (2003), pp 206-211. The Kwon article describes two lead-containing having the compositions, in weight percent, of:                (1) 89.1% PbO, 0.5% ZnO, 9.2% B2O3, 1.4% SiO2, 0.2% Fe2O3, 0.04% SnO2 and 0.1% BaO; and        (2) 72.2% PbO, 15.0% ZnO, 9.8% B2O3, 1.9% SiO2, 0.9% SnO2, 0.1% BaO and 0.1% TiO2.Lead-containing sealing glasses are commercially available from, for example, Ferro Corporation, Cleveland, Ohio who supplies Pb—B, Pb—Zn—B and Pb—B—Al.        
The basic requirements for a non-Pb (that is, Pb-free) sealing frit for plasma displays are that the Pb-free frits have compatible coefficients of thermal expansion (CTE, values of approximately 60-90×10−7/° C.) for the high strain point soda-lime type glasses used in the displays, and good flow at display sealing temperature (typically in the range of 450-550° C.). This invention is directed to lead-free phosphate glass compositions that meet these basic requirements and can be used as frits. These compositions are based on tin-zinc-phosphate and alkali-zinc-phosphate glass families.